The invention relates to a method for producing a superconductive element, in particular a multifilament wire, starting from a composite comprising a bronze matrix containing Cu and Sn, in which at least one elongated structure containing Nb or an Nb alloy, in particular NbTa, is embedded, whereby in a first step the composite is extruded at a temperature between 300° C. and 750° C., followed by cold or hot working and annealing steps in which the composite is elongated in parallel to the elongated structure and softened by a temperature treatment, called intermediate annealing further on, followed by a stacking step, in which a multitude of elongated composites from the preceding cold or hot working steps are bundled, the steps of extruding, elongating, annealing and stacking being repeated one or more times, followed by a final elongating process, including intermediate annealing processes, in which the composite is elongated to its final length, the superconductive phase being obtained by a heat treatment including a solid state diffusion reaction.
A method of this type is described in the article “Fabrication Technology of Superconducting Material” by H. Hillmann in “Superconductor Materials Science: Metallurgy, Fabrication and Applications”, ed. by S. Foner and B. Schwartz, NATO advanced study institutes series, B-Physics, Vol. 68, pp. 275-388, Plenum Press, New York/London, 1981.
Superconductive wires containing a superconductive Nb3Sn phase are typically produced by the powder in tube process (PIT-process), by the internal Sn diffusion method, or by the bronze route.
In the bronze route, a number of niobium (Nb) rods are inserted into a copper (Cu) and tin (Sn) containing bronze matrix. By repeated extruding, bundling and insertion into further bronze cans, a ductile wire with numerous Nb fibers embedded in a bronze matrix is obtained. Some pure copper is also introduced into the wire in order to improve its thermal conductivity. The wire is then brought into the desired shape, e.g. by winding the wire into a coil. Subsequently, the wire is annealed at a temperature of about 600-700° C. During this solid state diffusion reaction, Sn originating form the bronze diffuses into the Nb fibers and forms Nb3Sn, which has superconductive characteristics. The superconductive Nb3Sn phase is also called A15 phase.
Nb3Sn with low Sn content exhibits inferior superconductive properties, in particular a low critical temperature Tc and low upper critical magnetic field strength Bc2. Therefore, high and homogeneous Sn contents in the Nb3Sn phase are desired. The Sn content in the Nb3Sn phase can be increased by increasing the annealing temperature (=reaction temperature) and/or the annealing time (=reaction time). However, this also induces accelerated grain growth, which deteriorates the superconductive properties of the filament again.
The described Bronze route process is well established at the present day for bronzes containing up to ˜16 wt. % Sn (9.1 at. % Sn) in the unreacted wire, the fabrication method covering the largest part of the market. However, recent important progress in the two other techniques, the “Internal Sn” process and the Powder-In-Tube (or PIT) process has created a new situation: a further improvement of the critical current densities of bronze route Nb3Sn wires is mandatory to remain competitive in the market.
In U.S. Pat. No. 5,228,928, a method of manufacturing a Nb3Sn superconducting wire is described in which the Sn content of the bronze matrix is increased leading to an increased amount of the Nb3Sn phase thus improving the superconducting properties of the wire. The workability of the wire with increased Sn content is improved by dividing an intermetallic compound phase in the bronze into small pieces by cold or warm working at temperatures below the recrystallization temperature of the bronze matrix.
It is the object of the invention to provide a method for producing a superconductive element which has improved superconductive properties in a large volume fraction of its superconductive filaments, in particular a high critical temperature Tc and a high upper critical magnetic field strength Bc2, and which is mechanically stable enough for commercial applications such as magnet coils.